For horizontal placement consider real ball bearing fans, the should last longer than standard fans in horzontal placement,Fan Roundup #6: Scythe, Noiseblocker, Antec, Nexus, Thermalright
From the conclusions on the review,
The standouts for me are the Noiseblocker M12-S1 and the Scythe Gentle Typhoon 12, for amazingly low noise and excellent cooling performance at those low sound levels.
Now what you need to decide is what rpms you need, i had 5x 1850rpm version, and i had a fan controller and at 600rpm they are very quiet fans, i think i was able to drop them to 400 or around there before they stoped with the fan controller. My advise is go with the 800rpm version and undervolt it if you need it or if you cant find it, the 1150rpm version.